Generally, the projector is under restrictions of etendue determined by the light emission area and the divergence angle of a light source. The etendue restricts how much light from the light source can be used as projected light.
In other words, in the projector, if the value that is the product of the area of the light source and the angle of divergence is not made less than or equal to the value of the product of the area of a display element (e.g., liquid crystal panel) and the acceptance angle (solid angle) that is determined by the f-number of the projection lens, the light from the light source cannot be efficiently used as projected light. Thus, for example, even if a number of LEDs greater than a maximum number determined by the restrictions of the etendue are arranged in an array or a LED having a light emission area larger than a maximum light emission area determined by the restrictions of the etendue is used, the brightness of a projected image cannot be improved.
Patent Literature 1 discloses a 3-plate projector capable of expanding a color reproduction range without any reduction of light use efficiency caused by the etendue restrictions.
The 3-plate projector described in Patent Literature 1 includes first and second green LEDs whose peak wavelengths are different from each other, a red LED, and a blue LED.
The optical axis of the first green LED is orthogonal to that of the second green LED, and a dichroic mirror is arranged in a position where the optical axes of the first and second green LEDs intersect each other.
A green optical beam emitted from the first green LED is reflected by the dichroic mirror, and the reflected light is applied to a green liquid crystal panel. A green optical beam emitted from the second green LED passes through the dichroic mirror, and the passed light is applied to the green liquid crystal panel. The first and second green LEDs are both driven by current twice as large as rated current.
A red optical beam emitted from the red LED is applied to a red liquid crystal panel. A blue optical beam emitted from the blue LED is applied to a blue liquid crystal panel. The red and blue LEDs are both driven by the rated current.
A light flux passed through the green liquid crystal panel is orthogonal to a light flux passed through the red liquid crystal panel and, at their intersection, is orthogonal to a light flux passed through the blue liquid crystal panel. A cross dichroic prism is arranged in a position where the light fluxes intersect each other.
The cross dichroic prism synthesizes red image light from the red liquid crystal panel, green image light from the green liquid crystal panel, and blue image light from the blue liquid crystal panel. Image light synthesized by the cross dichroic prism is projected to a screen by a projection lens.
In the 3-plate projector, the red LED is lit for a period of one frame, and an image based on a red luminance signal is displayed on the red liquid crystal panel. Similarly, the blue LED is lit for a period of one frame, and an image based on a blue luminance signal is displayed on the blue liquid crystal panel.
The first green LED is lit, of first and second subframes constituting one frame, for a period of the first subframe, and an image based on a first green luminance signal is displayed on the green liquid crystal panel. The second green LED is lit for a period of the second subframe, and an image based on a second green luminance signal is displayed on the green liquid crystal panel. Accordingly, on the green liquid crystal panel, a first green image based on the first green luminance signal and a second green image based on the second green luminance signal are alternately displayed for each subframe.
According to the control, during the period of one frame, an image can be displayed by the four lights of red, first and second green, and blue. A color reproduction range in this case is wider than that when an image is displayed by the three color lights of red, green, and blue for the period of one frame.
The first green LED is driven by current twice as large as the rated current. Thus, when the ratio of the first subframe and the second subframe in one frame is 50:50, the amount of light acquired when the first green LED is lit for the period of the first subframe is approximately equal to that acquired when the first green LED is driven by the rated current to be lit for the period of the first frame. The second green LED is similarly driven by current twice as large as the rated current. Thus, an amount of light approximately equal to that acquired when the second green LED is driven by the rated current to be lit for the period of the first frame can be acquired. As a result, reduction in luminance caused by alternate lighting of the first and second green LEDs for each subframe can be prevented.
Further, the first green optical beam emitted from the first green LED and the second green optical beam emitted from the second green LED are applied on the same optical path to the green liquid crystal panel via the dichroic mirror. According to this configuration, as long as the light emission areas of the first and second green LEDs are within an area range determined by the etendue restrictions, most of the first and second green optical beams emitted from the first and second green LEDs are used as projected light.
Similarly, as long as the light emission areas of the red LED and the blue LED are within the area range determined by the etendue restrictions, most of the red and blue optical beams emitted from the red LED and the blue LED are used as projected light.